To my knowledge, you can't. I actually didn't even know the attatchment feature in the User CP was there until I looked just now. You can post pics in a thread by clicking the attatchment button under "Go Advanced." Or, you can hunt down the pic on the web and put the web address inside tags.

 

i think he's talking about an avatar (the pic below your username)

you gotta earn it by becoming a VIP member